1. Field of the Invention
This invention relates to a metal cord having a novel twisting structure used as a rubber reinforcing material in tires for a vehicle, a conveyor belt and the like as well as a pneumatic tire using such metal cords as a reinforcing material.
2. Disclosure of the Related Art
In the art, steel cords of so-called 1.times.4 or 1.times.5 twisting structure comprised of 4-5 filaments are widely used as a rubber reinforcement for a tire or the like. Recently, the use of steel cords having 1.times.2 twisting structure has started for attaining the weight reduction, low fuel consumption and low cost of the tires for vehicles, and also the use of single wire cord comprised of only wire filament is proposed.
However, when the cord of the twisting structure simplified by merely reducing the number of filaments such as 1.times.2 twisting structure, single wire cord or the like is used in the belt of the tire, if the excessive force is given in cornering, a so-called buckling deformation is caused in the ground contacting portion outside the belt. Hence the cord breakage is caused due to the deformation input. Therefore, there is adopted a procedure of adding a new reinforcing material to the edge portion of the belt in the building of the tire.
Since the use of such a new reinforcing material is contrary the purpose for attaining the weight reduction, low fuel consumption and the like of the tire, it largely decreases the merit of using steel cord of 1.times.2 twisting structure or single wire cord as a rubber reinforcement.
The steel cord of 1.times.2 twisting structure is proposed in Japanese Patent laid open No. 62-117893 and No. 62-234921.
However, such steel cords are very poor in the breaking property because the angle of steel filament itself too rises with respect to the longitudinal direction of the cord. Consequently, they are not used in the belt portion of the tire as they are.
Also, Japanese Patent laid open No. 2-229286 proposes steel cords of 1.times.2 twisting structure, in which forming ratios of two filaments are changed to periodically and alternately create portions of contacting the filaments with each other and portions of causing no filament contact (portion surrounded by rubber) in the longitudinal direction of the cord.
In the later article, it is described that when the steel cord formed by changing the forming ratios of the filaments is applied to the belt portion of the tire, it prevents penetration of water invaded from cut failure on the tread surface into the steel cord of the belt layer in the longitudinal direction of the cord. Thus, the cord breakage due to the occurrence of rust can be prevented.
In the steel cord of 1.times.2 twisting structure, however, it is not necessarily required to alternately create the filament contacting portions and the non-contact portions as described the above article because the steel cord of 1.times.2 twisting structure does not produce cord breakage and the like due to the occurrence of rust through penetration of water. Moreover, it is substantially impossible to embed such steel cords in rubber for the tire while alternately holding the contacting portions and the noncontact portions in the cord. Even if these cords are embedded in the rubber at such a state that the contacting portions and the noncontact portions are alternately existent in the longitudinal direction of the cord, the angle of the filament itself with respect to the longitudinal direction of the cord at the contacting portion is too large, so that the cord breakage may be caused.
On the other hand, the single wire cord is described in Japanese Patent laid open No. 57-198101 and No. 1-229704, which is a simple straight wire cord, so that the resistance to cord breakage is very poor unless a reinforcing member such as a layer material or the like is used.